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I know experience-based questions are a bit tricky on this site but isn't every theory based on common experience?
Every time I hear an air jetliner flying over at a high altitude it seems the engines are sort of humming. The sound isn't steady in time but instead little variations are to hear. They are not making the steady sounds of the engines you can hear when these are working on the ground.

What's the cause of this? The clouds?

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The humming sound is radiated by the rotating fan disc at the front of the engine. The outer periphery of the fan disc is traveling through the air at a speed close to that of sound, which causes the hum. The closer the blade tips approach the speed of sound, the louder the humming becomes.

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  • $\begingroup$ I'll wait a while to accept your answer, though I know you are right, thinking about it. Oh, why not now! $\endgroup$
    – Deschele Schilder
    Commented Sep 7, 2020 at 16:14
  • $\begingroup$ What about the comment made in the previous answer? $\endgroup$
    – Deschele Schilder
    Commented Sep 7, 2020 at 16:15
  • $\begingroup$ The same thing happens to jet (non-propellor) airplanes though. $\endgroup$
    – Deschele Schilder
    Commented Sep 7, 2020 at 16:17
  • $\begingroup$ @descheleschilder, that's because almost all jet engines have a huge fan as the first compressor stage; the rotating fan blades make the hum, $\endgroup$
    – niels nielsen
    Commented Sep 7, 2020 at 16:32
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At least some differences are caused by the Doppler effect, as the direction to the moving plane changes with time.

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  • $\begingroup$ It's not that the sound gets higher or lower when the plane passes. It's going too high to hear this effect (noticeable). It's more like the sound oscillates between a deep and high roaring of the engines. $\endgroup$
    – Deschele Schilder
    Commented Sep 7, 2020 at 7:56
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    $\begingroup$ If the planes are few km high is reasonable that the air in between moves. Moreover it might be related to motion of the plane (other than the above doppler eff) in terms of various reflections from building and soil features. $\endgroup$
    – Alchimista
    Commented Sep 7, 2020 at 11:09
  • $\begingroup$ Why don't you include that in your answer? $\endgroup$
    – Deschele Schilder
    Commented Sep 7, 2020 at 15:04

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